Rector's away on retreat (4)
I believe the answer is:
rout
'retreat' is the definition.
(I have seen 'Disorderly retreat' mean 'rout' so perhaps 'retreat' could also mean 'rout')
'rector's away on' is the wordplay.
'rector' becomes 'r'.
'away' becomes 'out' ('out' can be a synonym of 'away').
'on' says to put letters next to each other.
'r'+'out'='ROUT'
(Other definitions for rout that I've seen before include "Overwhelming victory" , "Defeat comprehensively" , "Debacle" , "Completely defeat" , "Runaway defeat" .)